In my opinion, there are two ways to read this -- two possibilities as to who the figure is. I think that it could be George Wilson or it could be Daisy (but only in Gatsby's imagination).
The eNotes summary of this chapter believes that the figure is that of George Wilson. This makes sense because, after all, George Wilson is the only person who actually makes his way towards Gatsby in the pool. George comes there to kill Gatsby because he thinks Gatsby killed Myrtle. He would probably have been "ashen" because he would have been thinking about killing Gatsby and then himself. That would make me pale, I think.
I think that this is the most likely answer. However, I sometimes think that this might refer to Gatsby fantasizing about what might have been between him and Daisy.
I imagine that Gatsby went out to the pool, maybe knowing he was going to die. He would have been thinking about what he most wanted in life (Daisy) and how life no longer really mattered if he could not have her. Maybe he saw a vision of her gliding through the trees... But I still think it's really George Wilson that he sees.
The entire quote:
He must have felt that he had lost the old warm world, paid a high price for living too long with a single dream. He must have looked up at an unfamiliar sky through frightening leaves and shivered as he found what a grotesque thing a rose is and how raw the sunlight was upon the scarcely created grass. A new world, material without being real, where poor ghosts, breathing dreams like air, drifted fortuitously about...like that ashen, fantastic figure gliding toward him through the amorphous trees."
This comes from chapter 8. Myrtle Wilson, Tom Buchanan's mistress, has been run over by a car. It was Gatsby's car, but Daisy was driving it. Daisy and Gatsby and Nick, Jordan and Tom had been driving back to Long Island after being in NYC where Gatsby forced Daisy to tell Tom she never loved him, and she really could not say that she never loved him. When they passed by Myrtle and George Wilson's garage, Myrtle, who had been locked upstairs by George, her husband, because George had learned Myrtle was having an affair, saw Gatsby's car from the window. She thought it was Tom and ran down to talk to him, ran in front of the car, and Daisy ran her over. George does not know WHO killed his wife, but Tom, who figures out what happened, comes upon the accident because he is driving behind Gatsby and Daisy. He has been trying to sell George a car and knows George, so he whispers something to him. Later, we find out that he no doubt told George that the car that hit Myrtle was Gatsby's car. George assumes that Gatsby is Myrtle's lover, not Tom.
So, the "ashen figure" is George. He has gone to Long Island to find the car. He figures out that it belongs to Gatsby. Gatsby, who has never used his pool all summer, decides to have one last swim before he lets his servants drain it, and Geroge is hiding in the trees, waiting for him. He rises up from the trees and kills Gatsby.
Great answers so far. I just have one other comment to add to the discussion. The entire quote, as given above, is from the narrator's--Nick's--point of view. It begins with the speculation "he must have felt." Nick wasn't there, nor were we, the readers. It's only a collection of suppositions and random news items which Nick puts together to narrate for us the probable order of events listed in both posts. Fitzgerald clearly wanted to offer enough "hints" to us that we would figure it all out--connect the dots, so to speak. Given that, it's evident that it was Gatsby's killer, George Wilson. However, the entire sequence of events, and certainly those which no one saw or reported, is primarily speculation.